The process of ion implantation is a critical manufacturing element used by the semiconductor industry. Implantation makes possible precise modification of the electrical properties of well-defined regions of a semiconducting work-piece by introducing selected impurity atoms, one by one, with a velocity such that they penetrate the surface layers and come to rest at a specified depth below the surface. The characteristics that make implantation such a useful processing procedure are threefold: First, the concentration of the introduced charged dopant atoms can be accurately measured by straight-forward integration of the incoming electrical charge delivered to the work-piece; secondly, the patterning of dopant atoms can be precisely defined using photo-resist masks; finally, the fabrication of layered structures becomes possible by varying the ion energy.
The ion species used for silicon implantation include arsenic, phosphorus, germanium, boron and hydrogen. The required implant energies range from below 1 keV (kilo-electron volts) to several hundred keV. Ion currents used range from microamperes to multi-milliamperes. Projecting to the future, demands are for greater productivity (elevated ion intensities); implantation at energies well below 1 keV; improved precision of uniformity and ion-incidence angle-control at the wafer.
During the last decade there has been an industry shift towards the use of D.C. ribbon-beams. This technology arranges that dopant ions arrive at a semiconducting wafer as part of a uniform-intensity beam that is organized into a long, small-height stripe that simultaneously implants uniformly the whole width of a semiconductor wafer. This geometry makes possible uniform implantation of a wafer during a single pass under the ribbon beam. The advantages of ribbon beam technology are substantial: (1) Batch implantation of multiple wafers and the use of large spinning discs is no longer necessary as the energy density at the wafer is low. (2) Wafers move slowly along a single linear path, avoiding issues of damage to delicate circuit components related to collision of heavy particles that arrive at the wafer surface.
U.S. Pat. No. 5,350,926 entitled “High current ribbon beam ion implanter” and U.S. Pat. No. 5,834,786, entitled “Compact high current broad beam ion implanter”, both issued to N. White et al., present aspects of ribbon beam technology. Implanters, generally designed according to these principles, are manufactured by Varian Semiconductor Equipment Associates of Gloucester, Mass.
Referring to FIG. 2 it can be seen that in a typical ribbon beam tool a first magnetic deflector directs wanted-mass ions through a mass-resolving aperture where unwanted species from the ion source are rejected. Downstream of this aperture the emitted fan-shaped beam, now comprising only wanted ions, is parallelized by a second magnet and transformed to the ribbon length needed for implanting a specific wafer diameter. A deceleration system beyond the mass rejection aperture is included to reduce the energy of ions arriving at the wafer; the purpose being to allow the use of ion source extraction energies that are best suited for efficient source extraction and high transmission efficiency through the mass-resolving aperture.
In a ribbon beam implanter the control of space charge is a central issue. These effects are manifest mainly downstream of the deceleration region and are particularly troubling in the region of the second magnetic deflector where the presence of a magnetic field makes it difficult for the beam potential to trap the necessary neutralizing electrons: Captured electrons have difficulty moving across the magnetic field lines but can easily escape to the poles unless some form of electron trapping is present. Also, there is evidence that electron temperatures grow within magnetic fields further increasing electron losses. Thus, as a consequence of inadequate neutralization, the boundaries of the beam tend to expand allowing ions to be intercepted at the magnet poles or at the walls of the vacuum chamber.
Space charge problems have been recognized since the days of the Manhatten Project's development of the Uranium Bomb. An historical review, including the impact of space charge on that project, has been written by William E. Parkins and published on page 45 of the March 2005 edition of the magazine Physics Today. Further background for these processes can be found in a book entitled ‘Large Ion Beams’ written by A. T. Forrester and published by John Wiley and Sons in 1988. The above referenced book presents data and calculations on pages 139 to 153 concerning the manner in which ions ‘peel away’ from the outside of a drifting low-energy ion beam. In addition, data is included concerning the difficulties of achieving space charge neutralization within magnetic fields and the manner in which the ion-beam potential is raised as it passes through a magnetic field. Other authors who discuss space charge effects include V. Dudnikov in U.S. Pat. No. 6,329,650 and F. Sinclair, et al. in U.S. Pat. No. 5,814,819.
The solution which provides at least partial neutralization of the effects of space-charge expansion depends upon the fact that the same electric field distribution that causes the boundaries of a positive ion beam to expand because of space-charge effects is also an electric field distribution that attracts negative ions or electrons towards the center of an ion beam. However, even when created within the beam potential itself, these electrons tend to concentrate near the center of the positive ion beam leaving peripheral regions somewhat short of electrons, causing a tendency for ions to ‘peel-away’ from the outer edges of a ribbon beam. This peeling effect will be accentuated by the fields generated between image charges at the surface of a narrow vacuum envelope and non-neutralized positive ions within the beam itself.
In the energy range above ˜15 keV interactions between fast beam ions and residual gas molecules usually provides sufficient secondary electrons that the space-charge density of the ion beam is largely neutralized. However, magnets whose focusing properties are satisfactory for deflecting ion beams having energy above ˜15 keV may not provide acceptable transmission in the energy region below 5 keV, due to the above space charge effects. Additional magnetic field components may be needed for compensating residual space charge effects and for improving beam transmission through magnetic fields, the central theme of the present patent disclosure.